1. Field of the Invention
The present invention relates to a zoom lens and an optical apparatus including the zoom lens, and is suitable for, for example, still cameras, digital cameras, video cameras, projectors, and the like.
2. Description of the Related Art
Recently, the size of image-pickup apparatuses (cameras), such as video cameras and digital still cameras, which include solid-state image pickup elements, has been reduced. Accordingly, there has been a demand for small zoom lenses having a wide field angle for use as optical systems in the image-pickup apparatuses.
In this type of camera, various optical elements, such as a low-pass filter and a color compensating filter, are placed between the rearmost portion of the lens and an image-pickup element. Therefore, a zoom lens used in this type of camera is required to have a relatively long back focus.
In addition, in a camera including an image-pickup element for obtaining a color image, to avoid shading of brightness and color, a zoom lens is required to have telecentricity on the image side.
A so-called retrofocus zoom lens including a lens unit having a negative refractive power and a lens unit having a positive refractive power in order from the object side to the image side is known as a wide-field-angle zoom lens having a long back focus and good telecentricity on the image side.
A three-unit zoom lens including a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power in order from the object side to the image side is known as a retrofocus zoom lens (U.S. Pat. Nos. 7,289,273, 6,618,210, 6,646,815, 6,646,817, and 7,027,379).
U.S. Pat. Nos. 7,289,273, 6,646,817, and 7,027,379 describe a small zoom lens having high optical performance which adequately corrects chromatic aberrations using a compound optical element obtained by laminating a resin layer on a lens made of glass.
Recently, so-called retractable zoom lenses have been used in many cameras to reduce the size of the cameras. In a retractable zoom lens, when no shooting operation is performed, distances between lens units are reduced to distances different from those set in a shooting operation, so that the amount of projection from the camera body can be reduced.
In general, when the number of lenses included in each lens unit of a zoom lens increases, the length of each lens unit along the optical axis also increases. Accordingly, even when the zoom lens has a retractable structure, the length of the zoom lens in the retracted state increases. Therefore, it is important to reduce the number of lenses in the lens units.
However, if the number of lenses included in each lens unit is simply reduced to reduce the length in the retracted state, it becomes difficult to correct aberrations like spherical aberration and coma aberration that affect monochromatic imaging performance. In addition, the kinds of glass materials that can be used are limited, and it can be extremely difficult to correct chromatic aberrations.
To reduce the number of lenses without degrading the imaging performance, various methods are used in which a lens surface included in the zoom lens is formed in an aspherical shape. However, in the case where an aspherical surface is formed, although aberrations regarding the imaging performance for a single color can be corrected, it is difficult to correct the chromatic aberrations, which are largely affected by the selection of the glass material.
In particular, in a retrofocus zoom lens, if the kinds of glass materials that can be used are limited to obtain a wide field angle, it is extremely difficult to adequately correct the chromatic aberration of magnification.
In addition, in the retrofocus zoom lens, the lens structure is asymmetric with respect to an aperture stop as a whole. Therefore, it is extremely difficult to obtain high optical performance over the entire zooming range while reducing the number of lenses to reduce the size of the zoom lens.
In particular, in the retrofocus zoom lens, to obtain high optical performance while reducing the size of the lens, it is important to adequately set the lens structure of a first lens unit having a negative refractive power to adequately correct variations in various aberrations including the chromatic aberrations during zooming. This is also important to reduce the size of the entire lens system.
An aspherical lens manufactured by laminating a resin layer or the like on a lens body and pressing a mold having an aspherical shape against the resin layer is often used in a zoom lens. However, in this type of aspherical lens, the chromatic aberrations are not positively corrected using the difference between the materials of the lens and the resin.